"The issue between Croatia and Slovenia is a heavy burden on the Western Balkans. If not solved soon, the whole Western Balkans momentum is killed," the source said, adding that "there is nothing [the commission] can do legally" to intervene.

Hungary, for one, welcomed Juncker's initiative.

"If you take a look at the US, Russia, they all have a Balkans strategy, but the EU doesn't have a Balkans strategy … the EU should be a stabilising force there," Zoltan Kovacs, the Hungarian government spokesman, said on Monday.

He said Montenegro was "ready to join" the EU and that Serbia "should be taken in as soon as possible - 2025 would be nice".

Spanish problem

The diplomatic push is to see the 28 EU leaders meet their six Balkan counterparts at a special enlargement summit in Sofia in May.

Spain, which does not recognise Kosovo's independence from Serbia, has threatened to spoil the show.

It said in an informal paper last month that the new Western Balkans strategy should "draw a clear distinction" between Kosovo and the five other Balkans aspirants.

"The concept of 'WB6' [Western Balkans 6] does not fit the enlargement dynamic. Kosovo is not part of the enlargement process," it said.

Four other EU members - Cyprus, Greece, Romania, and Slovakia - also do not recognise Kosovo.

But they have not rallied round Spain at this stage, while EU diplomats played down the importance of Madrid's lone intervention.

Greece came the closest to backing Spain.

"Athens supports the European perspective of the whole Western Balkan region, as stated in the Thessaloniki Agenda", a Greek diplomat told EUobserver.

The agenda, an old EU policy, was adopted at a special EU summit before Kosovo had declared independence in 2007, with Greece adding that "the principle of unanimity must be ensured regarding any change in the participation at these meetings".

Creative language

Other sources said EU Council chief Donald Tusk would have to compose an artful invitation letter and conclusions for the Sofia summit, which did not mention "states" or "heads of states" with regard to Kosovo, in order to keep Madrid happy.

But a diplomat from a central European EU state said: "We take this Spanish document as one of [just] many contributions to the discussion".

"The summit in Sofia will be about the future of the whole Western Balkans and its relations with the EU. We perceive the participation of Kosovo in this context," the diplomat said.

Another EU contact said Madrid's paper had been drafted before Catalonia, a Spanish region, voted to split from Spain in a referendum in December, and that it contained "nothing new".

A third contact said that unless the Catalonia crisis flared up before May, Madrid was unlikely to make a big fuss on Kosovo.

Turkey not invited

Spain's informal paper, seen by EUobserver, also mentioned the need to make nice with Turkey - the only other place with an EU membership perspective.

"Despite the current circumstances, Turkey must not be forgotten", Madrid said, referring to Turkish president Recep Tayyip Erdogan's authoritarian crackdown.

"It is our oldest enlargement partner, joined to us in a strong multidimensional relation. It is also both a key player in the region and a basic strategic ally of the EU," Spain said in its own geopolitics.

Erdogan has voiced interest in attending the Sofia summit.

But EU officials felt his coming would overshadow the event, with Tusk and Juncker aiming to meet him in a smaller "institutional summit" in February or March instead.

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After the European Commission presented its Western Balkans strategy last week, with a view of possibly integrating the region by 2025, some EU ministers were less enthusiastic after their first discussion of the new policy.

Opinion

Albania currently faces a serious crisis, which it would be foolish for all actors in the international community to ignore. Yet we must ask that our partners in Europe read Federica Mogherini's report carefully and recognise accomplishments.